Laser printers, scanners and photocopiers are widely known appliances for recording and/or printing images using laser (light) beams. Such appliances typically utilize a ROS system having a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated light beams across a recording medium (i.e., a photoreceptor, such as a drum or belt coated with a photosensitive material) in a line scanning direction (e.g., parallel to the drum's axis; also know as the scan direction) while the recording medium is being advanced in an orthogonal or “process” direction (e.g., along the drum's surface in a radial direction; also known as the cross-scan, slow scan, or sub-scan direction) such that the beam scans the recording medium in accordance with a raster scanning pattern.
Next-generation multiple beam scanners for laser printing will employ semiconductor laser arrays with large numbers of beams (up to 32 and higher). The advantage of utilizing a large number of beams is the increased bandwidth capability that will be used to increase speed and scan resolution by factors up to four and higher, and enable revolutionary printer features.
One problem associated with multiple beam scanning optical systems is that, due to the two dimensionality of the array, beams are offset in the scan direction. This results in the off-axis beams being collimated at non-zero angles. The non-zero angles require a larger polygon facet to capture all of the beams. For polygons with six or more facets the polygon size can become prohibitively large.
Another problem associated with multiple beam scanning optical systems is that low levels of bow, diffraction limited aberration correction, and wobble correction are required for good image quality. Referring to FIG. 12, “scan line bow” is a measure of distortion in the cross-scan direction from one end of the scan to the other. Bow may be calculated by taking the average of the cross-scan heights BH1 and BH3 at the extreme ends of the scan line, then subtracting it from the cross-scan height BH2 at the center of the scan line. In a multiple beam system, each light source emitting a beam has its own bow curve. As indicated in FIG. 12, an uppermost beam of the multiple beam system traces the upper scan line, and a lowermost beam traces the lower scan line. The maximum spacing between the uppermost beam and the lowermost beam anywhere along the scan line is shown as BSmax. The minimum spacing between the uppermost beam and the lowermost beam anywhere along the scan line is shown as BSmin. The difference between the maximum and minimum beam spacings, BSmax−BSmin, defines the “differential bow”. Diffraction limited optical systems are ones in which the optical aberrations are low enough that diffraction effects determine its performance. Wobble is caused by changes in the polygon facet tilt angle due to non-symmetry of the rotating polygon mirror, which causes the reflected beams to be displaced relative to their ideal cross-scan position on the photoreceptor. For manufacturability reasons, multiple beam scanning optical systems also require a reasonable degree of telecentricity, i.e., output beams with small cross-scan chief ray angles. Telecentricity ensures that the beam separation does not vary greatly with changes in the distance between the ROS and photoreceptor.
For conventional dual-beam systems, the beams are typically off-axis by 21 μm to 63.5 μm in the cross-scan direction at the photoreceptor plane. At these dimensions, near-telecentric systems with single-cylinder mirror wobble correction optics are sufficient to achieve low-bow, diffraction limited performance. However, for future systems that will employ two-dimensional laser arrays with up to 32 beams and higher, where the beams can be off-axis by as much as 500 μm in the cross-scan direction at the photoreceptor plane, current systems will be inadequate.
FIG. 2(B) is an unfolded, cross-scan view showing a conventional optical system disclosed in U.S. Pat. No. 6,833,939 (Ichikawa, incorporated herein in its entirety) that is capable of achieving negligible amounts of scan line bow and diffraction limited imaging for large beam separations. The optical system is positioned between a laser array LA and a photoreceptor PR, and includes a pre-polygon (laser to facet) optical subsystem made up of a collimator lens CL, an aperture stop AS, and a beam conditioning lens BC, and a post-polygon (facet to photoreceptor) optical subsystem including an Fθ scan lens, first and second concave cylinder mirrors M1 and M2, and an output window OW. Both the pre-polygon and post-polygon optical subsystems are input and output telecentric. The post-polygon optical system is afocal in that it is input and output telecentric. This explains why a single cylinder mirror wobble correction system will not work since an afocal system with the desired properties requires at least two elements.
The combination of a refractive cylinder lens and a cylinder mirror can produce an afocal system (see U.S. Pat. No. 5,512,949, Fisli, Grafton, Xerox Corporation, also incorporated by reference in its entirety). However, the performance of this optical system is inadequate for large beam separations.
Wobble correction is achieved in the optical system of FIG. 12 by imaging or nearly-imaging the facet PF onto the photoreceptor PR in the cross-scan direction. Because the facet is a real object, the afocal optical system must be of the Keplerian type. In a Keplerian configuration, the object and image is separated by twice the sum of the cylinder mirror focal lengths, 2 (f1+f2), and the distance from the second mirror M2 to the photoreceptor PR is equal to the focal length f2 of the second cylinder mirror M2. The magnification is equal to the negative ratio of the focal lengths, −f2/f1, where f1 and f2 are the focal lengths of the first and second cylinder mirrors M1 and M2, respectively.
With the optical system of FIG. 12, the cross-scan post-polygon magnification is typically less than −0.5. This means that the distance between the second cylinder mirror M2 and the photoreceptor PR will be smaller than the distance between the facet PF and the first cylinder mirror M1.
For beams that are off-axis by ±500 μm, the optical system of FIG. 12 is capable of achieving residual scan line bow levels below 0.1 micron at maximum beam separations up to 1 mm, and a telecentricity less than 0.1 mrad. The architecture may be able to obtain this kind of performance at higher beam separations.
While the optical system of FIG. 12 can be used in next generation scanning optical systems that employ two-cylinder mirror wobble correction optics, such two-cylinder mirror system has a short working distance D1 (i.e., distance between second mirror M2 and photoreceptor PR), which precludes its use in appliances that require a larger working distance to provide room for other subsystems that must be located close to the photoreceptor surface or to provide denser packing in print engines that utilize multiple optical scanning systems.
What is needed is a multiple beam optical system that facilitates the use of polygon beam deflectors having minimum facet size, and that also features low levels of bow, diffraction limited aberration correction, and wobble correction, a reasonable degree of telecentricity, and also provides a relatively large work space distance between the output window and the photoreceptor.